Also known as: Kerckhoffs's law, Kerckhoffs's desideratum
Kerckhoffs’s principle states that a cryptosystem should stay secure even if everything about it except the key is publicly known.1 In other words, the secrecy must live entirely in the key, never in the design of the algorithm.
How it works
The principle was articulated by Auguste Kerckhoffs in the nineteenth century as one of several design requirements for military ciphers, and later restated by Claude Shannon as the maxim “the enemy knows the system.” Its practical force is simple: assume your adversary has a full description of the algorithm, and design so that this knowledge alone gives them no advantage — the only thing they still lack is the key.
This is the direct rebuttal to security through obscurity, the idea that a secret method can substitute for a secret key. Hidden methods tend to leak, get reverse-engineered, or be independently rediscovered, after which a system whose safety depended on that secrecy collapses. Open, publicly reviewed algorithms — such as those used in symmetric and public-key cryptography — are scrutinised by many analysts and gain confidence precisely because their internals are known yet they remain unbroken.
Relevance to SDR
The principle is the right lens for the difference between encryption and obfuscation in radio systems. The open P25 and DMR air interfaces publish their framing and even their cipher choices, and their voice protection still holds because the key is secret — exactly as Kerckhoffs prescribes. By contrast, a scheme that relies on keeping its method hidden is mere obfuscation: once the method is recovered from public on-air data, anyone can reverse it. GopherTrunk’s reference work treats such schemes strictly clean-room — analysed only from observed signals and public documentation, never from any third-party source — and the Motorola P25 talker-alias encoding studied in issue #773 is an obfuscation of this kind rather than true encryption.
Sources
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Kerckhoffs’s principle — Wikipedia, for the requirement that only the key be secret and Shannon’s restatement. ↩